A phone company has a monthly data plan where a customer pays flat monthly fee of $30 and then a certain amount of money per gigabyte (GB) of data used on the phone. If a customer uses 3GB, the monthly cost will be $39. If the customer uses 8 GB, the monthly cost will be $54.

a. Find a linear equation for the monthly cost of the data plan as a function of x, the number of GB used.
C(x)=

b. Use your equation to find the total monthly cost if 10 GB are used.
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